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There have been significant improvements during the past decade in the worldwide ability to detect covert nuclear explosions equivalent to only a few hundred tons of chemical explosive, experts told a recent AAAS-organized discussion on Capitol Hill.

All but the most determined efforts at evasion likely would be spotted by a growing array of seismometers, radiation monitors, and other devices designed to detect nuclear blasts underground, underwater, in the atmosphere, and in space, they said.

In 2002, a panel of the U.S. National Research Council determined that an underground nuclear explosion with a yield of 1 to 2 kilotons (equivalent to 1000 to 2000 tons of TNT) could not be confidently hidden once a fully functional seismic monitoring system was in place as part of preparations for enforcement of the Comprehensive Nuclear-Test-Ban Treaty.

That treaty, adopted by the United Nations General Assembly on 10 September 1996, still is not in force because the United States and several other nations with nuclear technology have yet to ratify it.

Richard Garwin

But development of a sophisticated monitoring network has continued nonetheless. The International Monitoring System (IMS), operated by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) in Vienna, Austria, is now about 80% complete—with more than 260 facilities certified—and is much more capable than the system in place when the National Research Council made its projections in 2002.

The system, when fully built, will consist of 337 facilities worldwide that employ four monitoring methods:

Hydroacoustic: Six underwater hydrophone stations and five land stations that monitor the oceans for underwater explosions. Sound waves associated with explosions can travel thousands of miles underwater.

Infrasound: 60 surface stations that can detect ultra-low-frequency sound waves (inaudible to the human ear) emitted by large explosions.

Radionuclide: 80 stations that measure airborne radioactive particles associated with nuclear explosions (half of the stations also detect varieties of a noble gas called xenon that are associated with nuclear explosions). The stations are supported by 16 radionuclide laboratories.

“Technical capabilities have improved significantly in the past decade,” said physicist Richard Garwin, an IBM Fellow Emeritus and member of another National Research Council study panel that recently updated the 2002 report and reviewed technical issues related to the test ban treaty. He and other specialists on nuclear test monitoring spoke at a 24 September Capitol Hill discussion organized by the AAAS Center for Science, Technology and Security Policy.

The update from the Council (the principal operating agency of the National Academy of Sciences and the National Academy of Engineering) was released in March. It concluded there is now 90% confidence that the IMS seismic stations could detect an underground nuclear explosion well below 1 kiloton in most regions. The first-generation nuclear weapons that were used against Japan in World War II had yields of between 10 and 20 kilotons.

More than 2000 nuclear tests were carried out between 1945 and 1996, when the Comprehensive Nuclear Test Ban Treaty was opened for signature. The United States conducted 1032 tests, the Soviet Union more than 715, France more than 210, and the United Kingdom and China 45 each, according to the CTBTO. Three countries have broken the de facto moratorium on nuclear testing since 1996: India, Pakistan, and North Korea.

Lassina Zerbo

Under normal circumstances, a nuclear blast with a yield of 1 kiloton creates a seismic signal approximately equal to a magnitude 4.0 earthquake. Lassina Zerbo, director of the International Data Centre for the CTBTO, said there now is a 90% probability that at least three seismic stations in the monitoring system will pick up an underground explosion in the northern hemisphere comparable to a 3.5 magnitude earthquake and an explosion comparable to a 4.0 magnitude quake in the southern hemisphere.

The detection capability will continue to improve as more facilities are added in the southern hemisphere and elsewhere. “We’re continuing to install stations and improving our processing method,” Zerbo said, “and then we’ll certainly be much better than where we are today.”

In addition to the IMS facilities, there are thousands of seismometers and other sensors worldwide that can help pick up signs of a nuclear blast, including “national technical means” deployed by individual countries (such as the U.S. Atomic Energy Detection System operated by the Air Force) and seismometers used by hundreds of academic and governmental research institutions.

Paul Richards

Data from the many seismometers worldwide can be combined to provide clues on the location, size, and character of various explosive events, said Paul Richards, a professor of natural sciences at Columbia University’s Lamont-Doherty Earth Observatory. Even a nation such as North Korea, which shares no seismic data with outsiders, is surrounded with detectors in nearby countries. There are 24 high-capability seismic stations in South Korea, Richards said, and more than 1000 stations have been installed in China during the past decade.

“Currently, access to them [the Chinese stations] is not as good as one would like,” he said. “But certainly, to a subset of these stations, there is open access.” Nearby, Japan is perhaps the best-monitored country in the world, with about 2000 seismic stations.

“So for the question of what assets are available to monitor North Korea,” Richards said, “it’s just quite an amazing variety.”

Ray Willemann

Ray Willemann, director of planning for the Incorporated Research Institutions for Seismology (IRIS), noted that portable seismology instruments—provided by IRIS to scientists funded by federal agencies—give excellent baseline information on how seismic waves propagate through particular parts of the Earth. For example, use of portable instruments has allowed American researchers and their local partners to understand much better how seismic waves are distorted as they travel from sites in India and northwestern China to U.S. monitoring systems or the International Monitoring System, he said.

A state could try to elude detection by “decoupling” a small nuclear blast in a deep underground cavity where the amplitude of the vibrations through surrounding rock would be reduced. They also could try to mask the seismic waves from a nuclear blast by conducting the test near a working mine site where conventional explosions occur frequently. But the National Research Council study found that mine masking is a less credible evasion scenario now than it was at the time of its 2002 report because of improvements in monitoring capabilities.

With better regional seismic networks, improved understanding of the seismic background signals (from several hundred earthquakes and several thousand mine blasts that occur every day), and better calibration of seismic stations, the research council panel concluded that an evasive tester in Asia, Europe, North Africa, or North America would have to restrict a nuclear device’s yield to less than 1 kiloton—even if fully decoupled or mine-masked—to ensure no more than a 10% chance of seismic detection. Such evasion methods also would run the risk of detection by other means, such as human intelligence leaks by mine workers or cavity excavators.

At well-monitored locations, the yield would have to be even smaller—on the order a few hundred tons of TNT or less—to give hope of getting away with it, the study concluded. It did note that more work is needed to better understand the local geology in regions where seismic waves are strongly attenuated. Iran, much of Turkey, and other parts of the Middle East are regions of poorer propagation of seismic waves, according to the study.

Robert Werzi

Of course, seismology is only part of the international monitoring effort. Robert Werzi, senior expert on radionuclide technologies for the CTBTO, said about 80% of the radiation monitoring stations for the IMS are now operational, with sensors that can readily identify nuclear-related releases worldwide. Within one month after the tsunami-related nuclear disaster at the Fukushima power plant in Japan in March 2011, all of the IMS radiation monitors in the Northern Hemisphere detected radioactive particles from the plant and several stations in the Southern Hemisphere, including one at Rio de Janeiro, also detected material, Werzi said. He said experts have made promising advances in their ability, using computer models, to pinpoint the origin of atmospheric nuclear releases (whether from power plants or bombs) and to predict where they will travel over time.

Even with the best technology, Garwin said, “there’s always a level at which a tester can confidently test and not be detected” by seismic instruments. He mentioned the possibility of carrying out tests with yields of only a few kilograms TNT equivalent inside an artificial pressure vessel sufficiently strong to contain the blast wave and other byproducts of the explosion. But Garwin said the National Research Council panel concluded that an evader wouldn’t benefit much from such a small-scale test.

He also noted that rogue nations may want the world to know they are nuclear-capable and take no steps to hide their tests. That was the case for North Korea, for example, which tested nuclear devices in 2006 and 2009, each of which was promptly detected.

Continuing improvements in the number and sensitivity of monitoring tools should make evasive testing of nuclear weapons a formidable challenge, Richards said. Only “at very low levels of yield” could a state or group hope to escape notice, he said, and “the chief goal of the monitoring effort is to drive ever downward the yields of anything that might go undetected or unidentified.”